1. Field of the Invention
The present invention relates to the field of determining the contour of a target body for imaging. Specifically, the present invention relates to body proximity detection for use within a gamma camera (nuclear medicine camera)for medical imaging for ECT imaging operations and for total body imaging operations.
2. Prior Art
Gamma detection cameras, also called gamma cameras, are used for medical imaging of particular body tissues, organs, or bone that may otherwise not be available for examination. In a typical medical camera of this sort, a special gamma ray emitting radiopharmaceutical is injected into the body area of interest in front of the patient and the patient is then placed within the medical camera's imaging surface. As is well known, the radiopharmaceutical emits gamma rays which are then detected by the gamma camera as a series of scintillations from a specialized crystal layer. Before the gamma rays reach the crystal they travel through a collimator which allows only those gamma rays which travel along the collimator's orientation. A matrix of photomultiplier tubes is optically coupled to the crystal layer to receive the scintillations within the crystal layer and converts these scintillations into electrical signals indicating a spatial coordinate of the gamma ray interaction. By using computers and other processing equipment to manipulate and plot the signals from the photomultiplier tubes, an image of the organ containing the radiopharmaceutical can be obtained and displayed for examination and diagnosis. If this type of nuclear medicine camera system rotates around the patient, it is called a single photon emission computed tomography or SPECT system. The surface of the gamma camera which receives the gamma rays from the patient is called the imaging surface or the detector surface. Since the collimator of the gamma camera is the first or outermost layer of the gamma camera, the collimator surface is commonly referred to as the imaging surface of the camera.
In practice for an ECT scan a patient is placed horizontally into a central location while a gamma camera rotates (transaxial rotation) around a predetermined portion of the patient to collect a number of data (projections). The projections are reconstructed into traverse slices. This "ECT" rotation is orthogonal to the cranial-caudal axis of the patient. The resultant data slice is then a cross-section of the patient or target organ (or bone) at the predetermined location along the cranial-caudal axis of the patient. A total body scan is a different scan technique than the ECT scan. For a total body scan, the gamma camera moves (translates) along the long (cranial-caudal) axis of the patient usually at the anterior or posterior orientation along the patient and no rotation of the imaging surfaces is done during the translation for a total body scan. As the camera surface translates, it collects the radiated gamma rays from the area where the radiopharmaceutical is concentrated. In SPECT and total body imaging to obtain best quality images, it is desired to place the collimator as close as possible to the patient's outer surface. It is universally understood that when the collimator to patient distance is minimized better image resolution develops. Prior art systems have attempted to determine and utilize information about body profiles or contours of the patient in order to minimize the distance between the camera imaging surface and the patient. With the body profile data, the camera imaging surface is adjusted to arrive at the minimum distance. For this reason it would be advantageous to develop a system to constantly and automatically minimize the distance between the camera imaging surface and the patient during the ECT and total body scan process. The present invention allows for such capability.
Prior art systems of proximity detection, as described in U.S. Pat. No. 4,593,189, have placed a "web" or planar array of beams in a plane located just in front of, and parallel to the imaging surface of a gamma camera as shown in FIG. 1(A) and FIG. 1(C). FIG. 1(A) and FIG. 1(C) illustrate side and frontal views of a gamma camera scanning detector 18. As shown in the side view, there is a collimator surface 11, behind which is located crystal layer 12 and a matrix of receiving photomultiplier tubes 17. Located in front of the collimator 11 is a small reflection ring 15. This reflection ring 15 is used to create a dense set of beams which surround the front surface of the imaging surface of the gamma camera. FIG. 1(A) also illustrates an inward frontal view of the gamma camera detector 18 perpendicular to the imaging surface of the gamma camera 18. This view is looking into the imaging surface or collimator 11. As shown, reflector ring 15 is a circular ring with a U-shaped cross-section. There are two laser beam emitters which are directed angularly onto the reflection ring 15 causing a "web" or dense set of planar beams 10 to surround the imaging surface of the gamma camera. Because the beams are located in a plane parallel to and in front of the imaging surface 11 of the camera, they comprise the outermost portion of the camera detector 18 as shown in the side view of layer 15 and increases the collimator to patient distance.
Referring still to the prior art design of FIG. 1(A) and FIG. 1(C), in operation the camera detector 18 is mounted on a movement device or gantry. In an effort to minimize the distance between the collimator 11 and the patient, the camera detector 18 urges forward toward to the patient until a part of the patient impinges within the reflector ring 15. When this happens, most likely a beam of web 10 will become interrupted or weakened by the impinging object thus causing the camera detector to back away from the patient until the light beam is no longer interrupted. Using a feedback processor, the camera of this prior art design moves toward and away from the object during the ECT scan depending on whether or not a light beam of web 10 has been interrupted by the scan object. This prior art design is disadvantageous for a number of reasons. Most importantly, the mandatory presence of the detecting surface in front of the collimator reduces the resolution obtainable by the prior art system by increasing the scanning distance between the collimator and the patent. This system is not advantageous because it does not directly measure the profile of the target object. Also, this design requires a proximity detector for every camera detector in a multi-detector camera design. Also, this design does not actually calculate an object profile but performs multiple "trial and error" movements for proximity detection which are wasteful and could become haphazard. This approach can be very inefficient and time consuming while only giving minimal proximity information. It would be advantageous to be able to directly calculate the profile of a patient before the scanning operation is performed so that the gamma camera imaging surface 11 can be precisely adjusted to a close, predetermined, proximity with respect to the patient.
Another prior art design for proximity detection, as disclosed within U.S. Pat. No. 5,072,121, is illustrated within FIG. 1(B). This is a side view of the overall camera structure showing the side view of the circular gantry as the outer circle 26. The gamma camera detectors (not shown) of this design are located above and below the target object patient 25. The body of patient 25 is shown in a cross-sectional view surrounded by a side view of gantry 26. Surrounding the patient, and located in a circular ring near the gantry structure 26 are a series of emitters 20 which emit a light beam arc toward corresponding detectors 21 and 22 which are also located on the same circular ring. Each emitter 20 illuminates the patient with a light beam and thus creating a shadow 27 of the patient on the circular ring where the patient interrupts the beam. The emitters are sequentially pulsed so that there is a different shadow for each emitter as well as an illuminated portion where the beam is not interrupted. A series of detectors located on the circular ring detect the location of each shadow. A sequence unit controls the sequential firing of a series of these emitters located around the patient and therefore a series of shadow regions are produced depending on the characteristics of the patient. By calculating all the shadow positions for each emitter, some contour information can be configured by the prior art system.
The prior art design illustrated in FIG. 1(B) is disadvantageous because of the plurality of emitters and detectors required for the design. Along the ring 26 there must be at least a series of many detectors and many emitters which is costly and complex to manufacture and implement. Also, the resolution of the contour data obtainable by such a system is limited by the number of detectors 21, 22 placed on the outer ring 26. For each emitter and detector series only the shadow position (along the ring 26) of the object can be known. Also, patients having an asymmetrical body profile, such as patients having breast removal, may generate false profile data as the asymmetrical nature of the object 25 will cause shadowing which will hide important features of the true body contour under this prior art system. Also, partial profiles are not obtainable by direct measurement within this prior art system because shadows, not data profile points, are detected. What is needed is a system that can directly measure the body profile from a radial position at each point along the patient. Using such a system a true patient profile and contour can be determined and collected thus improving image quality. The present invention offers such capability.
Prior art system as described in U.S. Pat. No. 4,445,035 utilizes ultrasound emitters and detectors for proximity detection by placing four ultrasound emitters on the edge of the imaging surface of the ECT camera. The prior art system of this type measures the difference in time between the emitted ultrasound signal and an echo signal reflected from the target to determine a distance between the camera surface and the target. The prior art design is disadvantageous because proximity measurements are determined at each of the ECT imaging surface, which is a variable position reference point. Therefore, each time the ECT imaging surface moves in circular positions during the ECT scan cycle, new proximity measurements are required. This means that extra motions are required since the detector must move to the maximum diameter since there is no prior knowledge of the patient contour. What is needed is a system which does not require re-measuring the proximity data for a given target profile each time the camera imaging surface is moved through an ECT movement. The present invention offers such a system. In addition, this prior art system is not able to be readily constructed utilizing ultrasound technology when the medium surrounding the patient is air since air is not a good conductor of an ultrasonic signal. For this reason, the prior art system as disclosed may not be readily implemented with current technology and apparatus. This reference indicates substituting laser beams for the ultrasound emitters but fails to provide disclosure of a system capable of measuring the difference in time between the incident and reflective light beam and translate this difference into a spatial position value. Assuming such a system could be implemented, under current technology the radial resolution could only be determined to a crude +/-10 cm. Resolution at +/-10 cm would not offer any image quality improvement. This reference fails to disclose a system for measuring proximity information using infrared emitter device, receiving optics utilizing angular calculations. The present invention offers a system that may be readily implemented using currently available technology.
Therefore, it is an object of the present invention to provide a means for high resolution direct measurement of the data points constituting the contour of a patient imaged by a medical camera. It is an object of the present invention to advantageously utilize this profile data to construct a body contour of the patient. It is furthermore an object of the present invention to utilize the body contour data to control the proximity of the medical camera imaging surface so that the distance between the patient and the camera imaging surface is minimized thus improving overall image quality of the resultant display. It is furthermore an object of the present invention to utilize the body contour database for attenuation correction to further improve image quality. It is also an object of the present invention to accomplish the above using a system which does not require overly complex and costly structures and/or operations; it is an object of the present invention to operate in real-time. Other objects of the present invention not herein specifically mentioned will become clear according to the detailed description of the present invention to follow.